1. Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) optical communications system, and more particularly, to an apparatus and method for analyzing the spectrum of an optical signal using etalon to monitor a change in the wavelength of a wavelength division multiplexed (WDM) optical signal and the optical signal-to-noise ratio (OSNR) of the WDM optical signal.
2. Description of the Related Art
Monitoring of WDM optical signals in optical transmission systems is a key technique in wavelength division multiplexing (WDM). The monitoring of an optical signal of each channel as to whether the optical signals operate normally is required to obtain high reliability in optical transmission systems.
Measuring the OSNR of each channel, a change in the wavelength, and the number of wavelengths, which is essential for channel monitoring techniques, is performed by obtaining amplified spontaneous emission (ASE), and the output and wavelength in each channel.
In the prior art, a method of transmitting a dither signal to each channel, a method of using an arrayed waveguide grating (AWG) or a tunable filter which are optical devices, and other commercialized spectrum analyzer techniques are applied to achieve the above measurement.
Precise measurement of a peak value in a wavelength and the OSNR is essential for wavelength monitoring. However, the method of transmitting a dither signal to each channel and monitoring an optical signal in a receiving terminal using a phase locked loop (PLL) cannot obtain a wavelength value. The method using optical devices such as an AWG, a tunable filter, or the like, which can obtain even wavelength values, is quite costly. An optical spectrum analyzer or a multiple wavelength meter, which are used for optical spectrum analysis, is commercialized but expensive, so it is not suitable for channel monitoring. Therefore, there is an increasing demand for a spectrum analyzer which is simple and can be applied to monitor the channels of a WDM optical signal.
A spectrum analysis technique is the fundamental principle for achieving channel monitoring of a WDM optical signal. In existing spectrum analysis techniques using a Fabry Perot tunable filter, the thickness of etalon is varied by an electrical signal, thus deteriorating the accuracy and requiring a controller for variable control.
U.S. Pat. No. 5,825,792 for a Wavelength Monitoring and Control Assembly for WDM Optical Transmission Systems to Villeneuve et al discloses a Fabry-Perot etalon structure that provides feedback loop back to the laser source to control the laser source. What is needed is the use of such an etalon structure that interacts with the output of a fiber optic multiplexed signal to discern the spectral components and the optical signal to noise ratio of various wavelengths that may be traveling through an optical fiber.